This invention relates generally to dual-duplex, four-wire data communications systems.
More particularly, this invention pertains to devices, systems and methods for effectively canceling differential transmission delays associated with data communications across a pair of communications loops.
A complex issue that arises in dual-duplex, four-wire data communications systems is the separation (occurring at the sending end) and reconstruction (occurring at the receiving end) of data between the two wire pairs (loops). The incoming data to be transmitted must be split (usually evenly) between the two wire pairs at the input to the transmitters, and a single output data stream must be reconstructed from the two received data streams at the output of the receivers. The reconstruction process is made more complicated in a four wire (A and B loop) system if there are differential time delays introduced by each of the loops, such that data symbols that are time aligned at the transmitters in the central office transceiver are not time aligned when they arrive at the receivers in the remote transceiver.
Two approaches for separation and reconstruction of transmitted data used in the prior art are: (1) transmission of data in blocks or frames on each wire pair; or (2) symbol-by-symbol interleaving of data between the pairs, where a “symbol” is a unique token conveying information. For example, in quadrature amplitude modulation (QAM), where the data signal is both phase and amplitude modulated, each unique combination of amplitude and phase is a “symbol.” In the first prior art approach, significant bandwidth, referred to as framing overhead, may be required to allow correct de-framing of the data at the receiver. In the second approach, the framing overhead may be avoided by carefully measuring delays during training but this increases the complexity of the training sequence itself. In either of these prior art approaches, interleaving or framing data buffers must be used in the transmitter, receiver, or in both. The use of such buffers introduces undesirable latency into the system.
What is needed, then, is a data communications device and method that can cancel the effects of differential transmission delay in a dual-duplex, four-wire data communications system without increasing framing overhead, and without significantly adding to system latency.